Put the caster sugar and water in a saucepan, bring to the boil and heat to 121C.
Put the egg yolks in the bowl of an electric mixer and whisk, slowly pour in the hot syrup and whisk until thick and cool.
Gradually add the softened butter while still whisking. When all of the butter has been incorporated, continue to beat until light and aerated.
Add the salted caramel and beat until fully incorporated.